Pointing, printing, digitizing and similar devices are concerned at various levels with detecting a peripheral device in relation to a fixed reference point. The precise position sensing arrangement which is chosen depends on the particular task which the device is to achieve.
Relative motion sensing is used to control a cursor, for example, by means of a mouse incorporating a ball/sensor mechanical arrangement. This type of device detects incremental displacements transmitted to the sensors by the balls movement and thus the motion of the mouse in relation to the fixed surface can be used to control the position of an on-screen cursor. Such a technique however, does not provide absolute position information as there is no fixed reference point and each time the mouse is lifted and replaced on the mouse surface, there is no way of detecting its location.
Non-contact techniques for measuring relative displacement also exist. Optical devices using such techniques employ a CCD sensor which images a fixed, patterned surface. For example, the movement of an optical mouse is detected by imaging the movement of elements of a fixed surface pattern or texture to provide a translation vector and thus x and y cursor displacements.
These relative position measurement devices require a specific surface upon which to operate. In the former case, a rolling surface is used. In the latter, an optically patterned surface.
To extend relative position measurement techniques to arbitrary surfaces, methods have been developed which exploit the interaction between an incident and a reflected light beam which is shone onto a surface.
An example of this is U.S. Pat. No. 6,330,057 B1 to OTM Technologies Ltd. The technique provides a vector output based on heterodyne or homodyne detection of non-doppler, non-speckle image signals derived from changes in the phase and/or the amplitude of reflection from an optical surface. This technique allows any suitable optical surface to be used as the reference surface, in particular paper. It also allows the possibility of communicating translation signals to a computer system by moving a suitable object in the field of view of the stationary device such as by means of a gesture.
Absolute position measuring techniques fall into the broad category of devices which rely on a fixed “position-encoded” surface which is imaged in order to determine the absolute location of the imaging system and thus device in relation to the surface. These systems use a “glyph bed”. A glyph bed is an array of visible markings which encode absolute locations on the surface. Depending on the encoding technique, a glyph can encode a position on a logical page area which is extremely large. For example, WO 0126032 A1 to Anoto A B implements a glyph array aligned on a virtual grid with the positions of glyph marks in relation to the grid representing data values. WO 0126032 A1 describes a 4-bit cyclic encoding algorithm capable of encoding an extremely large logical area. Other examples of absolute position encoding systems include U.S. Pat. No. 5,852,434 to Sekendur which describes an absolute position measuring system whereby the position location is encoded into monolithic optically readable symbols arrayed on a grid on a page.
In both cases, a pen device is used which incorporates an imaging system in addition to a writing function. WO 0126032 A1 shows an example of such a device. The pens imaging system records the data encoded in the glyph or glyph bed in the field of view of the pen as strokes or pen “clicks” are executed. The time-varying absolute pen position, or (x, y, t) data are recorded by an associated computer.
A significant problem in absolute optical position measurement systems is obscuration of the glyph bed. This can result from overprinting with infrared opaque inks, damage to the paper and other affects which damage or obscure the position encoding glyphs. Obscuration causes the pen to lose sight of the glyph bed and if error correction or interpolation is not successful, stroke capture will be incomplete and/or error-laden. This problem can, to an extent, be overcome by restricting overprinting of human-readable matter to infrared transparent inks so that the pen can see through the overprinted areas. However, this solution is not ideal as it can introduce complications in terms of ink delivery during the printing process and may limit the available colour selection and production techniques.
Effective obscuration can also occur where the glyph bed is printed at a lower than normal resolution. In such a situation it is possible that the transient error-rate of stroke detection is sufficiently high to produce positioning errors and subsequent stroke data errors.
This problem is inhibiting the progress and adoption of these technologies and a solution to this difficulty is therefore highly sought-after. The invention attempts to overcome or at least ameliorate some of the abovementioned difficulties by providing an absolute non-contact optical absolute location system which is highly error-tolerant and resistant to obscuration effects in glyph detection. The invention also provides an improved optical non-contact absolute position measurement system which is suitable for low glyph resolutions.